System for detecting and amplifying modulated electrical variations



On. 27, 1936. w. R. FERRIS 2,058,432

SYSTEM FOR DETECTING AND AMPLIFYING MODULATED ELECTRICAL VARATIONS Filed NOV. 29, 1935 W. B FEB/3A5 ATTORNEY INVENTOR Patented Oct. 2 7, 1936 UNITED STATES PATENT OFFICE SYSTEM FOR DETECTING AND AMPLIFYING MODULATED ELECTRICAL VARIATIONS Warren R. Ferris, East Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware This invention relates to systems for detecting and amplifying modulated electrical variations such as radio signals and the like. More particularly, the invention relates to a vacuum tube circuit arrangement for providing approximately linear detection of a modulated signal voltage and amplification of the demodulated signal.

An object of the invention is to provide new and improved vacuum tube apparatus for detecting a modulated signal voltage and for amplifying the demodulated signal voltage. According to the invention, the detector portion is designed to give approximately linear detection and the amplifier portion is designed to efficiently amplify the demodulated output of the detector.

Another object of the invention is to provide a linear detector for the detection of the modulated signal voltage, which will operate with its input grid at a negative potential with respect to its cathode to thereby provide a high input impedance.

Still another object of the invention is to provide means in a detector and amplifier system ofthe type described, for automatically supplying a negative grid bias to the amplifier tube of such value as to permit high input signals of low percentage modulation to be applied to the grid of the detector without overloading the system, and to provide sufiicient response at low values of percentage modulation to operate a power amplifier or to operate directly into a loudspeaker or other output device with a high degree of efficiency.

Still another object of the invention is to connect the amplifier to the output of the detector in such a manner that the variation in the potential drop across an impedance in the output circuit of the detector may be applied between the grid and cathode of the amplifier to automatically bias the amplifier for eflicient operation without overloading.

Still other objects of the invention will become apparent on reading the following specification and appended claims in connection with the accompanying drawing which illustrates an approved form of the invention and a modification thereof.

In the accompanying drawing in which the same reference numerals are used to indicate like parts, Fig. 1 is a circuit diagram of apparatus assembled in accordance with an approved form of the invention, and Fig. 2 is a modification thereof. 7

In the modification of the invention illustrated by Fig. 1, a thermionic valve Ill of the three electrode type is connected in the circuit to function as a detector. A second thermionic valve II which may be, for example, of the three electrode type or of the type containing a plurality of grids, is connected to function as an amplifier of the demodulated output of the detector. The detector tube It] includes an anode I2, a cathode I3 and a grid Id. The cathode is preferably heated by a suitable heater element such as is indicated at [5. The amplifier tube l I likewise contains an anode IS a cathode H and a grid l8. It may also contain additional electrodes such as the grid I812. The cathode of the amplifier is also heated by a suitable heater element IS.

The detector input circuit i. e., the grid-cathode circuit of the tube It], includes a variable tuned circuit comprising an inductance I9 and a variable condenser 20. The tuned circuit receives energy from any suitable source such as the antenna circuit 2| which is coupled to the input circuit in any suitable manner such as by means of the primary winding 22. The input circuit to the detector also includes a resistor 23 shunted by a fixed condenser 24.

The anode-cathode circuit of tube It], includes a resistor 25 shunted by a condenser 26, and the resistor 23 shunted by condenser 24. Thus the resistor 23 is common to both the input and output circuits of the detector. The direct current path through the output circuit of the detector includes both resistors 25 and 23 connected in series.

The system is grounded by a ground connection at the grid end of the resistor 23. This causes the grid of the detector to be at ground potential, considering D. C. potentials, and the cathode to be positively biased with respect thereto.

The value of the resistor 23 is preferably sufficiently large so that the grid M has a negative potential with respect to the cathode equal to approximately times the potential difference between the anode and cathode, where mu is the amplification constant of the tube at out off. For example, if the resistor 23 is of the order of 1 megohm and mu is about 9, a plate supply voltage of 100 volts will divide so that approximately 90 volts appears between the plate and cathode and 10 volts from cathode to grid. The plate current must, of course be 10 micro-amperes to produce this voltage distribution and if the plate current of the tube is shown to be 10 microamperes by a static measurement at Ep= volts and Eg=10 volts the potential distribution is in equilibrium.

If an alternating signal Voltage of say 5 volts peak is applied between the grid and ground, the equilibrium of the potential distribution is destroyed. The applied voltage will cause a considerable increase of plate current on the positive half cycle and a smaller decrease on the negative half cycle. The excess of current charges the condenser 24 and raises the positive voltage of the cathode l3 so that more current flows through resistor 23 and eventually a dynamic equilibrium is again established. The eventual increase in cathode to ground potential is approximately equal to the alternating potential applied between ground and grid. If the alternating potential is slowly varied (modulated) the instantaneous potential across resistor 23 will follow the envelope of the wave quite faithfully thus providing a demodulated voltage. In order to obtain efficient amplification of the demodulated voltage the amplifier H is connected as shown. The grid N3 of the amplifier l i is directly connected with the cathode l3 of the detector l0 and the cathode ll of the amplifier is directly connected with the anode l2 of the detector. The input circuit of the amplifier thus comprises the output circuit of the detector. The output circuit of the amplifier is completed through any suitable load such as the sound reproducer 2?, a source of potential 28 and the resistor 25 shunted by condenser 2t. The resistor 25 is common to both the input and output circuits of the amplifier.

The voltage across the resistor 25 should be higher than the Voltage across the resistor 23 in order to insure that the cathode ll of the amplifier will be at a positive potential with respect to the potential on the grid I8 which is maintained at the same potential as that of the oatho'de of the detector tube. This arrangement will provide a negative bias on the grid l8 of the amplifier with respect to its cathode ll. It will also provide a positive potential on the anode [2 of the detector with respect to its cathode [3.

The desired voltage across resistor 25 may be obtained by proper choice of the resistor 25 and of the amplification factor of the tubes. The condenser 26 should be considerably larger than the condenser 24 in order to prevent a change in alternating potential across the resistor 25' during operation of the system. Satisfactory operation of the system has been obtained by using a condenser of micro-micro-farads at 24, a condenser of 8 microfarads (electrolytic) at 26, a resistor of 1 megohm at 23, and a resistor of 5000 ohms at 25. A voltage of 225 volts was used for the source of potential 28. The amplification factors of the triodes l0 and H were about 14.

When a signal is applied to the grid of the detector tube ID the voltage on the cathode of the detector tube (and on the grid of the amplifier tube II) will increase as explained hereinbefore. This tends to cause the amplifier tube to pass more plate current, but, of course, causes the potential of the cathode of the amplifier tube to rise according to' the drop in resistor 25. As a result there is very little change in the operating conditions of the second tube. However, if the input signal is varied (modulated) at a rate such that the by-pass condenser 26 prevents a change in the potential across resistor 25, the grid-cathode voltage of tube H will follow this change'andthe. tube will act as. an amplifier. As

the effect of a steady (unmo-dulated) signal upon the operating conditions of the amplifier tube is not great, the percentage modulation of the sig nal is unimportant, the principal effect being due to the modulation envelope of the wave.

In a modified form of the invention an amplifier of the push-pull type may be used instead of the single tube H of Fig. 1. Fig. 2 illustrates a circuit diagram of apparatus using a push-pull amplifier. In this modification the detector tube Hi and its input circuit including resistor 23 and condenser 24 are the same as in the modification illustrated by Fig. l. The output circuit of the detector tube is also the same as in the modification illustrated by Fig. 1 as it is completed through a resistor 25 shunted by a condenser 26, connected in series with a resistor 23 shunted by a condenser 24.

Instead of having a single amplifier tube l, two amplifying tubes 30 and 3! are connected in pushpull arrangement. The input circuit of tube 30 is completed throughthe output circuit of tube iii in the same manner as the modification illustrated by Fig. 1. The input circuit of tube 3| is likewise completed through the output circuit of tube Iii. The input circuit of the tube 3| also contains an audio transformer comprising the coil portions 32 and 33. The coil portion 32 which is connected. across the resistor 23 by means of a condenser 34 comprises aprimary Winding for transferring the potential drop across the resistor to the input circuit of tube 3!. Condenser 34 prevents the coil portion 32 from forming a direct current path around resistor 23 and it also functions to isolate the grid of tube 3| from the direct current potential at the grid end of the resistor 23. The output circuits of tubes 30and 3! are both completed through a load transformer 35, a source of potential 28, and resistor 25 shunted by condenser 26. The operation of the modification illustrated in Fig. 2 is similar to the operation of the modification illustrated in Fig. 1.

While I have illustrated and described a preferred form of the invention itis to be understood that various modifications can be made thereto without departing from the spirit of the invention. For example, the electrodes of the detector and amplifier tubes l0 and l I may be placed in a single evacuated vessel and certain of the electrodes may be combined to reduce the total number of electrodes. The amplifier tubes may also be of the tetrode or pentode type. Various other modifications may also be made to the apparatus and circuit arrangement by those skilled in the art without departing from the spirit of the invention or the scope of the appended claims. Itis, therefore, to be understood that the invention isnot to be limited to the exact circuit arrangement illustrated and described in the foregoing specification, but only by the scope of the appended claims.

What I claim is:

1. A detector-amplifier system for modulated radio frequency signals comprising a detector tube, an amplifier tube, a pair of resistors of different values connectedin series, connections connecting said resistors in a common circuitcon stituting the output circuit for the'detector' and the input circuit of the amplifier, and connections connecting one of said resistors in a lead common to the input and output circuits of one of said tubes and the other resistor in a lead common to the input and output circuits of the other tube.

2. A detector-amplifier system-comprising a detector, an amplifier, a-pair of resistors, one of said resistors having a higher ohmic resistance than the other, connections connecting said resistors in series in a circuit constituting the anode-cathode circuit of the detector and the grid-cathode circuit of the amplifier, and connections for connecting the resistor having the higher ohmic resistance in the anode-cathode circuit of the detector.

3. A detector-amplifier system comprising a detector, an amplifier, a resistance common to the anode-cathode and grid-cathode circuits of the detector, a condenser shunting said resistor, a second resistor, said second resistor having a higher IR drop than that of the first resistor, said second resistor being common to the anode-cathode and grid-cathode circuits of said amplifier,

and connections connecting said two resistors in series in a circuit constituting the anode-cathode circuit of said detector and the grid-cathode circuit of. said amplifier.

4. A detector-amplifier system comprising a detector tube, an input circuit for said detector tube, a resistor in the cathode lead of said input circuit, a condenser shunting said resistor, an output circuit for said detector tube, said output circuit including said resistor, a second resistor in said output circuit having an IR drop large compared to the IR drop of the first mentioned resistor, a condenser shunting said second resistor, an amplifier tube, a direct current connection between the grid of the amplifier tube and the cathode of the detector tube, a direct current connection between the cathode of the amplifier tube and the anode of the detector tube, and an output circuit for the amplifier tube, said output circuit including said second resistor.

5. A system for the detection and amplification of modulated radio frequency signals comprising a detector, an amplifier, a resistor common to the anode-cathode and grid-cathode circuits of said detector, a resistor common to the anode-cathode and grid-cathode circuits of said amplifier, connections connecting both of said resistors in series to provide a circuit constituting the anode-cathode circuit of said detector and the grid-cathode circuit of said amplifier, a second amplifier, connections connecting the second mentioned resistor in the anode-cathode circuit of the second amplifier, and other connections connecting both of said resistors in the grid-cathode circuit of said second amplifier.

6. A detector-amplifier system comprising a detector tube, a pair of amplifier tubes connected in push-pull, a resistor common to the anodecathode and grid cathode circuits of said detector, a condenser shunting said resistor, a second resistor common to the anode-cathode and gridcathode circuits of, said push-pull connected amplifiers, a condenser shunting said second resistor, and a direct current path through both of said resistors connected in series, said direct current path comprising the anode-cathode circuit for said detector and the grid cathode circuits for both of said amplifiers.

7. A system for demodulating modulated electrical variations comprising an amplifier tube, a detector tube, input and output circuits for said tubes, a pair of resistors each having one end grounded and the other end connected respectively to the cathodes of the amplifier and detector, connections connecting said resistors in the input circuit of. said amplifier tube and in the output circuit of the detector tube, and connections connecting one of said resistors in both the input and output circuits of said amplifier tube.

8. A detector-amplifer system comprising a detector having a resistor in its cathode lead, an amplifier having a resistor in its cathode lead, a direct connection between the detector cathode and the amplifier grid, and a direct connection between the detector anode and the amplifier cathode.

9. A detector-amplifier system comprising a detector having a self-biasing resistor connected between its cathode and ground, an amplifier having a self-biasing resistor connected between its cathode and ground, a direct current connection between the detector cathode and the amplifier grid, and a direct current connection between the detector anode and the ungrounded end of the amplifier self-biasing resistor.

10. A circuit arrangement comprising a linear detector, a self-biasing resistor common to both input and output circuits of said detector, an amplifier having its grid and cathode connected across the space discharge path of the detector output circuit, and a second resistor included in the output circuits of. both the amplifier and the detector.

WARREN R. FERRIS. 

